1. Field of the Invention
This invention relates to methods for recovering tar sands by establishing communication between an injection well and a production well penetrating a tar sand bed.
2. Description of the Prior Art
There are a number of known bitumen containing tar sand reservoirs scattered around the world. One of the largest of these is the deposit located in the Athabasca region of Alberta, Canada. The invention disclosed herein will operate in any tar sand bed; however, since the Athabasca tar sand deposit is well known the following discussion will mention it specifically.
The Athabasca tar sand deposit has a lateral area of several thousand square miles. The bitumen or oil bearing sandstone reservoir is exposed at ground surface is some areas of the deposit. Where this phenomenon occurs, open pit mining operations may take place. In these operations, oil and sand are separated in a plant. The greatest part of the deposit, however, is covered with overburden which can range up to 1000 feet in thickness. Where substantial overburden occurs, the deposit cannot be economically mined by open pit methods. Consequently, researchers in the field have worked toward developing an in situ method suitable for recovering the oil. The oil sand is mainly comprised of water wet quartz grains. The oil or bitumen is located in the interstices between the water sheathed grains and actually forms the matrix of the reservoir since the quartz grains are not in contact with one another.
The oil present in and recoverable from the Athabasca tar sands is usually a rather viscous material ranging in specific gravity from slightly below one to about 1.04 or somewhat greater. At a typical reservoir temperature, e.g., about 48.degree. F, this oil is a plastic material having a viscosity exceeding several thousand centipoise. At higher temperatures such as above about 200.degree. F, this oil becomes mobile with viscosities of less than about 343 centipoises. At reservoir temperatures then, it is evident that the oil cannot be pushed through the formation to a production well using conventional means such as a pressure gradient.
Thus, researchers have devised means for unlocking the subterranean tar sand so as to recover the contained oil. Most of these investigations have been concerned with converting the oil to a less viscous state so that it can be driven to and recovered from production wells using conventional pumping or gas lift methods. Many of these procedures are designed to heat the reservoir with steam or hot hydrocarbons so as to render the bitumen mobile. Other procedures involve spontaneously emulsifying the oil to form an oil and water emulsion and can be moved to production wells.
In these methods, the prior art teaches drilling production and injection wells into the formation and fracturing the tar sands horizontally to establish communication between the wells. After communication is established, the prior art methods then pump steam or an emulsifying fluid through the fracture system. One problem with these systems, is that the emulsion cools as it moves away from the hot zones surrounding the injection well and as it cools, the oil again solidifies to form an impermeable block in the fracture system. Another problem is that the tar sand even at temperatures from 40.degree.-50.degree. F is a plastic solid material which under the enormous overburden will slowly flow into the fracture zone thereby blocking it.
Thus, these prior art processes are hampered by the fact that the fracture will not maintain itself for long periods of time even though propped with extraneous materials. It is an object of our invention to present a method for fracturing a tar sand formation whereby the fracture is given rigidity and permanence.